Add a slave db handle for the SQLAlchemy backend.

Adds an option for a slave DB connection for scaling
purposes. Read queries can be sent here to spread out
db load a bit. This patch adds a new configuration option:
slave_connection.

Implements: blueprint db-slave-handle
Change-Id: I5cc5e0696e3fcd8609ee6b5466b0377ca52b8b58
DocImpact
Merge "Add a slave db handle for the SQLAlchemy backend."